1. Field of the Invention
The invention relates in general to elevator systems, and more specifically to the enhancement of traction in traction type elevator systems to permit the use of a lighter elevator car and counterweight.
2. Description of the Prior Art
Reduction in the dead weight of the elevator car is desirable from a cost viewpoint. In addition to cost savings in the elevator car itself, it enables savings to be made in associated items such as the counterweight, ropes, guide rails, safety, and traction drive machine. A lighter elevator car reduces power requirements from the associated building, as lower peak torques and thus currents are required. A lighter car also results in less mechanical loading on the building.
A limiting factor in reducing the weight of the elevator car and counterweight however, is the tractive force between the ropes and the drive sheave. Sufficient tractive force or traction must be available over the complete range between no-load or empty car, to full load, for the desired acceleration and deceleration rates.
The tractive force between the ropes and sheave is governed by the relationship: EQU T.sub.1 .div.T.sub.2 =e.sup..mu..theta.
where:
T.sub.1 =rope tension on car side of drive sheave PA1 T.sub.2 =rope tension on counterweight side PA1 e=the base of natural logarithms PA1 .mu.=the effective coefficient of friction between the rope and drive sheave for the groove geometry employed PA1 .theta.=angle of wrap or contact between the ropes and drive sheave.
In the prior art, sheave grooves are undercut to increase the effective coefficient of friction .mu., but only so much can be done in this regard as the resulting increased pressures on the ropes and sheave grooves shorten both rope and sheave life. Sheave grooves have also been lined with a treaded, elastomeric material in order to increase the effective coefficient of friction .mu.. Other expedients relate to increasing the angle of wrap by going to a double wrap, and even to a 270.degree. wrap. Increasing the wrap, however, increases the bending forces in the ropes, reducing rope life, and the 270.degree. wrap erodes the sides of the grooves because of the turning and tilting of the drive components in order to prevent interference between the ropes.